1. Field of the Invention
The invention relates to a precharging device of a hydraulic brake system of a vehicle for selective imposition of brake fluid under pressure on a brake assembly, by means of a charging unit.
2. Description of the Prior Art
Precharging devices of the type with which this invention is concerned are used for targeted braking of individual vehicle wheels in the context of an electronic stability program (ESP) for the sake of stabilizing a vehicle that is unstable and is spinning. A targeted braking of individual vehicle wheels is done by active brake pressure fixtures by means of a brake assembly in the form of a hydraulic block. To achieve effective stabilization of an unstable, spinning vehicle, relatively high dynamics must be assured by active brake pressure fixtures, so that a targeted braking of the wheels of the vehicle can be done at an operationally optimal instant and with a desired intensity. However, the dynamics of the active brake pressure fixtures is affected by several ambient conditions, especially the prevailing ambient temperature at the time and the ambient air pressure, which is also variable. A low ambient temperature and a low ambient air pressure leads to relatively poor brake pressure buildup dynamics. The brake pressure buildup dynamics are also affected in terms of their effectiveness by the composition, and in particular the water content, of the brake fluid used. To achieve the most persistently uniform, effective brake pressure buildup dynamics, which should essentially be independent of the prevailing, variable ambient conditions, in an electronic stability program (ESP) so-called xe2x80x9cprechargingxe2x80x9d is performed, in which a brake fluid is pumped from a supply tank or from a master cylinder to a hydraulic block of the vehicle by means of a precharging device. By means of such precharging, a precharge pressure is generated on the intake side of a return pump located in the hydraulic block. In known precharging devices, such precharging is done for instance by means of a precharging pump, intended especially for the purpose, or by means of so-called xe2x80x9cbooster prechargingxe2x80x9d, that is, by means of a driver-independent actuation of a brake booster of the vehicle. Disadvantageously, the known precharging devices are relatively complicated in terms of their construction.
The precharging device of the invention is characterized in that for activating the imposition of the brake fluid, the charging unit is operatively connected to a fuel supply system of the vehicle. The precharging device offers the advantage that a pressure generating unit already existing in the vehicle and used to supply fuel to the driving engine of the vehicle can be additionally used for precharging for active braking interventions. Thus additional, separate pressure generating units needed solely for creating active brake pressure fixtures are omitted, making a compact and structurally relatively simple design of a precharging device possible. Utilizing a fuel pressure that is preferably permanently present in the fuel supply system, the charging unit can be activated in a reliable, operationally favorable way for brake fluid imposition on a given brake assembly.
Advantageously, the charging unit is embodied as a piston unit. A piston unit is relatively simple to make and is suitable for producing a reliable operative connection with the fuel supply system of the vehicle. Because of the design flexibility of a piston unit, an adapted, operationally favorable precharging can be generated.
The fuel supply system is advantageously embodied as a fuel injection device. Both modern Otto engines and modern Diesel engines have fuel injection systems in which relatively high fuel pressures prevail, regardless of the engine operating state at the time, in specially provided fuel chambers (such as a so-called xe2x80x9crailxe2x80x9d, in Diesel engines). From these fuel chambers, when the applicable driving engine is in operation, a controlled imposition of fuel on the individual cylinders takes place, in which cylinders combustion occurs in a known manner. A fuel injection device is thus especially suitable, because of the fuel pressures that are typically always present during operation of the internal combustion engine, for establishing an operative connection with a charging unit, which in particular can be embodied as a piston unit.
In a preferred embodiment, the piston unit has a fuel piston and a brake fluid piston operatively connected to the fuel piston by means of a connecting rod, which are displaceable in an adapted cylinder by means of the fuel injection device as a function of a switchable fuel imposition. In this way, it is possible to divide the piston unit into a fuel operation region and a brake fluid operation region, and different operating pressure can prevail in the two operation regions.
Advantageously, the piston unit has a fuel pressure chamber, which is operatively connected by means of a switchable valve to the fuel injection device for selective fuel imposition. A switchable valve, which preferably has a plurality of different functional positions, is especially well suited to producing a reliable, flexible fuel imposition on the fuel pressure chamber and thus for effective precharging.
Advantageously, the piston unit is operatively connected to a fuel reservoir. The fuel reservoir is part of the fuel injection device and is preferably embodied in the form of a so-called rail. It is under a relatively high operating pressure. This operating pressure of the stored fuel serves on the one hand to provided controlled fuel imposition on the corresponding driving or internal combustion engine, and on the other, it can be used simultaneously for activating and generating a precharging operation.
In an alternative embodiment, the piston unit is operatively connected to a fuel pump means. The fuel pump means is likewise part of a fuel injection device and is preferably embodied as a so-called xe2x80x9ccommon-rail pumpxe2x80x9d. Once again, the relatively high fuel pressure generated can be used both for imposing fuel on the applicable internal combustion engine and for performing a precharging operation.
Advantageously, the operational face of the fuel piston is smaller than the operational face of the brake fluid piston. By a selection of the particular size of the operational face of the fuel piston and of the brake fluid piston, it is possible to obtain a hydraulic boost (or reduction) by means of the piston unit. In this way, the relatively great differences in pressure between the fuel region and the brake fluid region of the piston unit can be compensated for relatively simply.
Advantageously, the piston unit has a brake fluid pressure chamber, which is operatively connected to the brake assembly and to a brake fluid tank by means of a respective supply line that has an associated check valve. The check valves serve to prevent deflection of the brake fluid piston, and thus also of the fuel piston, caused by an incident operating brake pressure during braking performed by the driver, and to assure correct filling of the brake fluid pressure chamber.
Advantageously, the valve is embodied as a switchable control and/or regulating valve. A switchable control and/or regulating valve is especially well suited to establishing a flexible, reliable and correct operative connection between the charging unit and the fuel supply system.
In a preferred embodiment, the valve has a functional position that separates the piston unit from the fuel injection device. In such a separating functional position of the valve, additional or alternative security is obtained in terms of the prevailing operating position of the fuel piston at the time, and thus also of the brake fluid piston, especially during driver-performed braking (increased driver braking pressure).
Preferably, the valve together with the fuel pump means and/or the fuel reservoir forms a structural unit. In this way, a compact, easily installed structural unit is obtained, and the requisite high-pressure lines for establishing an operative connection between the valve and the fuel supply system can advantageously be kept relatively short. The piston unit is preferably disposed in the engine compartment of the vehicle.